Mobile x-ray inspection system for large objects

ABSTRACT

A device for inspecting a cargo container such as a motor vehicle or freight pallet, with penetrating radiation. A source of penetrating radiation is mounted on a moveable bed, thereby allowing a beam of penetrating radiation to sweep the large container. At least one detector is also mounted on the bed, either on the side of the source or on a boom, so that, as the beam is scanned across the container, the container and any contents of the container are characterized by transmitted or scattered radiation, or both.

The present application is a continuation of U.S. patent applicationSer. No. 09/072,212, filed May 4, 1998, now U.S. Pat. No. 5,903,623currently allowed, itself a continuation of Ser. No. 08/799,533, filedFeb. 12, 1997, issued as U.S. Pat. No. 5,764,683, all of which areincorporated herein by reference. The present application claimspriority from U.S. provisional application No. 60/011,495, filed Feb.12, 1996, also incorporated herein by reference.

The U.S. Government may have a paid-up license in portions of thisinvention and the right in limited circumstances to require the patentowner to license others on reasonable terms as provided by the terms ofContract No. N39998-97-C-5209 awarded by the U.S. Department of theNavy, and Grant No. 93-G-003 awarded by the U.S. Department ofTransportation, Federal Aviation Administration.

FIELD OF THE INVENTION

The present invention relates to the inspection of large containers bymeans of x-ray radiation, and more particularly to the x-ray inspectionof cargo containers by means of transmitted and back scatteredradiation.

BACKGROUND ART

The interdiction of illicit drugs, explosives, and other contraband isan important goal of law enforcement. To that end, a variety oftechnologies have been developed and deployed for the non-intrusiveinspection of containers not readily susceptible to visual scrutiny fromthe outside. The non-intrusive aspect of these inspection techniques isimportant; the great majority of containers do not carry contraband, andthe public would not long tolerate the delays, disruption (and in somecases damage) of property, and invasions of privacy that would occur ifinvasive inspection means were commonly used. Non-intrusive inspectionis typically non-destructive and can usually be accomplished faster thanintrusive inspection, hereby increasing productivity of inspectors.Increased productivity means more containers inspected and morecontraband interdicted.

Among non-intrusive inspection methods, x-ray imaging in its many formshas been a proven technology capable of detecting a variety ofcontraband. X-ray systems have been based on transmission imaging in anyof a variety of implementations: cone-beam (fluoroscopes), fanbeam,flying-spot, multi-projection configurations; dual-energy imaging;computed tomography; as well as on imaging incorporating the detectionof x-ray radiation scattered in various directions.

With only a few exceptions, x-ray imaging systems for contrabanddetection have operated within the source-energy range of 70 keV to 160keV. Since the penetrating power of these sources is limited, inspectionis limited to relatively small parcels and containers. A few large,expensive, high-energy transmission imaging systems have also been builtfor the inspection of large containers or vehicles. These systemstypically operate within the range of 6 MeV to 12 MeV, requireheavily-shielded, fixed installations, and provide no means todistinguish organic materials.

Radiant energy imaging with a scanning pencil beam is the subject ofU.S. Pat. No. 3,780,291. The creation and use of images from scatteredx-radiation in conjunction with direct transmission images is thesubject of U.S. Pat. No. 5,313,511. These patents are herebyincorporated herein by reference. Neither of the foregoing references,nor any prior art known to the inventors herein, discloses or suggestshow x-ray images, in a sense defined below, may be formed of cargocontainers on the scale of motor vehicles or railroad cars using asource of penetrating radiation and detectors mounted on a mobileplatform.

SUMMARY OF THE INVENTION

The current invention extends the role of inspection by penetratingradiation to encompass large containers and over-the-road vehicles, andat the same time retains the advantages of discriminating amongmaterials as provided by backward, forward, or sideward scatter imaging.

In a preferred embodiment, the invention provides a device forinspecting a cargo container with penetrating radiation. The device ofthis embodiment has a bed moveable along a first direction having ahorizontal component, a source of penetrating radiation, mounted on thebed, for providing a beam, a motorized drive for moving the bed in thefirst direction, and at least one detector mounted on the bed and havinga signal output so that the beam is caused to traverse the cargocontainer as the bed is moved and each detector provides a signal forcharacterizing the cargo container and any contents of the cargocontainer. In further embodiments of the invention, the source ofpenetrating radiation may be an x-ray source, and the at least onedetector may be a backscatter detector, a transmission detector, asidescatter detector, or a forward scatter detector. The beam may be apencil beam scanned repeatedly along a second direction having avertical component.

In a further embodiment, the device also has a boom, movably linked tothe bed. The boom has a storage position, substantially fixed withrespect to the bed, and an operation position that is transverse to thefirst direction, as well as an end region. A beam catcher is attached tothe end region of the boom for impeding the further passage ofpenetrating radiation in the second direction when the boom is in theoperation position. The back scatter detector has upper and lowerelements, the upper element being movably mounted with respect to thelower element, so that in a first position the upper element istransversely disposed with respect to the lower element and in a secondposition the upper element is substantially collinear with respect tothe lower element. The source of penetrating radiation may provide apencil beam that is scanned repeatedly over an angle of regard, theangle having an orientation with respect to the horizontal, and thedevice also includes a steering arrangement for steering the orientationof the angle of regard with respect to the horizontal.

Instead of fitting the boom with a beam catcher, the boom may bealternatively provided, at its end region, with a transmission detectorattached to respond to penetrating radiation transmitted through thecontainer, so as to provide a signal for forming a transmission image ofthe container. In further embodiments, the source of penetratingradiation includes an x-ray tube operating at a voltage in substantialexcess of 200 kV, and may operate in a region of approximately 450 kV.

In a further embodiment, the invention provides a method, for producingan x-ray image of a large object, utilizing a device such as describedabove, along with an arrangement for processing the signal from thesignal output of the detector to form an image of the object.

In a further embodiment, the invention is a scatter image of an object.The image is formed by (a) providing a device such as described above,then (b) using the motorized drive to move the device past an object soas to cause the object to be scanned along two dimensions; and (c)processing the signal from the signal output of the detector to form animage of the object. The object may, for example, be a motor vehiclehaving at least two pairs of wheels. The object may also, for example,be a trailer, a railroad car, a sea cargo container, an air cargocontainer, or a freight pallet.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will more readily be understood by reference to thefollowing description taken with the accompanying drawings, in which:

FIG. 1 is a plot of the x-ray interaction coefficients of two materials,heroin and iron, over the range of x-ray photon energies from 10 to 500keV.

FIG. 2A is a perspective view of a device for inspecting a largecontainer with penetrating radiation in accordance with a preferredembodiment of the invention.

FIG. 2B is a side view of a further embodiment of a device forinspecting a large container with penetrating radiation in accordancewith the invention.

FIG. 3 is a top schematic view of the layout of the system shown in FIG.2B, as configured for transport.

FIG. 4 is a side elevation schematic view of the layout of the systemshown in FIG. 2B.

FIG. 5A shows the cargo container inspection system of FIG. 2B, asdeployed for inspection of a full-sized tractor-trailer.

FIG. 5B shows the cargo container inspection system of FIG. 2B, asdeployed for inspection of a passenger van.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

As used in this description and in the appended claims, a “cargocontainer” is a receptacle for the storage or transportation of goods,and includes freight pallets as well as vehicles, whether motorized ordrawn, such as automobiles, the cab and trailer of a truck, railroadcars or ship-borne containers. The term “cargo container,” as usedherein, further includes the structures and components of thereceptacle.

The invention described herein serves to characterize materials whichmay be contained within a cargo container and thus not readilysusceptible to visual scrutiny. The characteristics of a material whichmight be the object of non-invasive inspection and which lend themselvesto detection using the device and method taught by the inventioninclude, but are not limited to, electron density, atomic number, massdensity, linear dimensions and shape. These characteristics are unveiledby taking advantage of the various physical processes by whichpenetrating radiation interacts with matter. Penetrating radiationrefers to electromagnetic radiation of sufficient energy per photon topenetrate materials of interest to a substantial and useful degree andinclude x-rays and more energetic forms of radiation. The interaction ofsuch radiation with matter can generally be categorized as eitherscattering or absorption processes. Both types of process remove x-rayphotons from a collimated (i.e., directional) beam; scattering processesdo so by deflecting photons into new directions (usually with loss ofenergy), while absorption processes simply remove photons from the beam.

Conventional transmission imaging measures the total beam attenuation asa function of position on the image plane, without discriminatingbetween absorption and scattering processes. The total beam attenuationis described by a parameter called the mass attenuation coefficient, ascommonly employed by persons skilled in the art of x-ray inspection. Themass attenuation coefficient is a characteristic of a particularmaterial at a specific x-ray photon energy, and is independent of theimaging geometry. As such, it is the sum of individual coefficients (or“cross sections”) for each relevant physical process, each of whichvaries differently with x-ray energy and with the atomic number (Z) ofthe interacting material.

In the range of photon energies useful for penetrating and screeningcargo containers, the scattering contribution is dominated byincoherent, or Compton scattering, and the absorption contribution isdominated by the photoelectric effect. The cross sections for Comptonscattering and photoelectric absorption vary with both the atomic numberof the material and with the energy of the x-ray photon, but in verydifferent ways. The photoelectric absorption decreases very rapidly withincreasing photon energy, and increases very rapidly with increasing Zof the material. The Compton scattering cross section changes veryslowly with energy and is only weakly dependent on atomic number. Thesefunctional relationships are illustrated in FIG. 1 for two relevantmaterials (iron and heroin). The Compton scattering cross section 10 fora low-Z material such as heroin and the Compton scattering cross section12 for iron are nearly identical over the energy range of interest, andshow little variation with energy. However, the photoelectric crosssections 14 and 16 are very different for the two materials, and bothvary rapidly with energy. For heroin, the Compton scattering crosssection 10 dominates the photoelectric cross section 14 for all x-rayenergies above 20 keV. In contrast, for iron the photoelectric crosssection 16 dominates for energies up to about 120 keV. Over the rangefrom 60 keV to 400 keV, the total absorption coefficient (the sum of theCompton and the photoelectric coefficients) varies by less than a factorof two for heroin but by about a factor of 13 for iron. Such differencesin scattering and absorption characteristics between low Z materials,characteristic of organic materials, and high Z materials,characteristic of most metals and their alloys, are typical and providethe means to differentiate between these two classes of materials.

Conventional transmission x-ray images-simply provide a map of theattenuation characteristics of the inspected object for the fullspectrum of the x-ray beam. It should be noted that images may bedirectly displayed in graphic format for the visual inspection of humanoperators, but need not be so displayed. As used in this description andin the appended claims, the term “image” refers to any multidimensionalrepresentation, whether in tangible or otherwise perceptible form orotherwise, whereby a value of some characteristic is associated witheach of a plurality of locations corresponding to dimensionalcoordinates of an object in physical space, though not necessarilymapped one-to-one thereonto. Thus, for example, the graphic display ofthe spatial distribution of some feature, such as atomic number, in oneor more colors constitutes an image. So, also, does an array of numbersin a computer memory or holographic medium. Similarly, “imaging” refersto the rendering of a stated physical characteristic in terms of one ormore images.

Backscatter imaging in which the x-rays scattered by a material in agenerally backward direction are employed offers several uniqueinspection capabilities and operational features. (1) Taken alone, it isa one-sided imaging modality: images can be obtained even when theobject is accessible from only one side, or, the object is too thick tobe penetrated radiographically. (2) Because the scatter signal falls offquite rapidly with increasing depth into the object, backscatter imageseffectively represent a “slice” of the object characteristic of the sidenearest to the x-ray source; this image is frequently useful even when atransmission image representing the same scanned area is hopelesslyconfused by image clutter. (3) The underlying physical phenomenon thatleads to scattered radiation is the Compton effect. Low atomic number(low Z) materials, which encompass organic materials, interact withx-rays principally by Compton scattering. Narcotic drugs, being amongthe densest of organic materials, tend to produce the brightestsignatures in a backscatter image, as do organic explosives, makingbackscatter imaging a useful imaging modality for bomb or drugdetection. (4) Alignment requirements of the x-ray beam with detectorsor collimation devices are less exacting than for transmission imagingthereby enabling rapid deployment in a wide range of inspectionscenarios.

To the extent that Compton scattering removes x-ray photons from thex-ray beam, it contributes to the attenuation map image produced bytransmitted x-rays. However, a scattered photon that is removed from onebeam path projection and scattered into another contributes only anunwanted background or “fog” in the transmitted x-ray image. Incone-beam imaging geometries (e.g., film radiography), the scatteredbackground can be a serious problem, and can, in fact, dominate over thetransmitted x-ray image data.

It is known to persons skilled in the art of x-ray inspection thathigh-Z and low-Z materials may be separately identified by measuringtotal attenuation at two different photon energies. This is the basisfor the dual-energy systems in widespread use for airport baggageinspection.

Another method to image low-Z materials is backscatter imaging. Thetechnique relies upon the direct detection of photons which have beenCompton scattered. An image is created that is separate and independentof any transmission image that may be produced at the same time. Sincethe photoelectric absorption cross section is small for organicmaterials, they interact almost entirely through Compton scattering,producing relatively large scatter signatures. Metals, on the otherhand, interact almost exclusively by photoelectric absorption, so thattheir scatter image signature is comparatively small. The backscatterimage directly reveals organic materials such as drugs or explosives.

Flying-spot technology makes possible the acquisition of images usingdetectors specifically positioned to collect the scattered x-rays. In atypical flying-spot system, a thin “pencil beam” of x-rays is rapidlyand repetitively swept through a source-centered, vertically-oriented“fan” of beam paths that are arranged to intercept the object underinspection. At the same time, the object is moved at a constant, slowerspeed along a path perpendicular to the fan, on a horizontally movingconveyor belt for example. In this way, the pencil beam is made totraverse the object in point-by-point raster fashion, and the entireobject is scanned as it passes through the fan plane over a periodranging from a few seconds to a few minutes depending upon the length ofthe object.

Although the total scan time is seconds to minutes in duration, theactual exposure time of any part of the scanned object is only the timeit takes for the pencil beam to sweep across that pixel. That exposuretime is typically in the range of 8 to 64 microseconds, depending on thedesign and the application, and yields an entrance exposure to thescanned object of only tens or hundreds of microroentgens. This low doseto the object also means that there is little radiation available toscatter into the environment, so the doses to operators and otherbystanders is correspondingly low. Separate, large-area detectors aredeployed adjacent to the beam plane on the x-ray source side of thescanned object, and with their active surfaces oriented toward thescanned object. These detectors need only provide a large solid anglefor collection of scattered radiation; no critical alignments arerequired. In this location these detectors respond to x-rays which arescattered generally back toward the source from the object.

Typically, the transmission x-ray signature of organic materials isrelatively weak in comparison to that of higher density, higheratomic-number materials such as metals. Since the x-ray transmissionimage is a result of interactions throughout a path through the entireobject, larger and more complex objects such as cargo containers producemore confusing transmission images. Under these circumstances, even thepresence of small amounts of metal and normal expected organic materialscan produce extremely cluttered images masking the sought-forcontraband. Image interpretation then becomes an overwhelming task.Frequently, most of the useful information is obtained from thebackscatter image alone.

In a preferred embodiment of the present invention, the cargo containerinspection device uses flying-spot x-ray imaging and backscatter imagingtechnologies. Equipment costs can be reduced if backscatter imaging isused to the exclusion of other modes of x-ray interrogation of thecontents of the cargo container.

Referring now to FIG. 2A, a perspective view is shown of a cargocontainer inspection system, designated generally by numeral 20, inaccordance with a preferred embodiment of the invention. Cargo containerinspection system 20 is shown deployed for inspection of passenger cars22 and 23. FIG. 2B shows a preferred embodiment of the invention. Withreference to FIGS. 2A and 2B, a truck 24, typically 35′ long×8′wide×10′6″ high, houses and supports the x-ray inspection equipment,ancillary support and analysis systems, and a hydraulic slow-speed drivemechanism to provide the scan motion. Truck 24 serves as both theplatform on which the mobile system is transported to its intendedoperating site, and a bi-directional translation stage, otherwisereferred to herein as a “bed,” to produce the relative motion requiredduring a scan. Chopper 26 is used, in accordance with flying spotgeneration techniques known to persons skilled in the art, to scan beam28 of penetrating radiation recursively in a vertical direction.Radiation scattered by the contents of the cargo container, shown hereas passenger car 23, is detected by x-ray backscatter detectors 30. Boom32 allows beam stop 34 to track beam 28 as truck 24 advances in scanningpast the cargo container. Beam stop 34 is also referred to as a “beamcatcher.” In addition or alternatively to beam stop 34, an x-raytransmission detector may be mounted in opposition to beam 28, trackingbeam 28 on the distal side of the scanned cargo container.

Referring now to FIG. 3, a top schematic view of the layout of thesystem shown in FIG. 2B, is depicted as configured for transport. FIG. 4is the corresponding side elevation, additionally showing the detectorsin one of two available deployed positions. The modular componentscomprising the cargo container inspection system are: the penetratingradiation source assembly 40; x-ray high voltage generating subsystemincluding high voltage power supply 42 and high voltage tanks 44;backscatter detector modules 30, comprised of an upper bank 46 ofdetectors and a lower bank 48 of detectors; detector electronics module50; and operator's console 52. The dashed position of upper backscatterdetector banks 46 indicate the position for inspection of cargocontainers. The x-ray source 40, high-voltage power supply 42, andpositive and negative high-voltage tanks 44, are all in accordance withordinary practice in the art of x-ray generation. In a preferredembodiment of the invention, a 450 kV x-ray tube is employed.

FIG. 5A shows a cargo container inspection system 20, in accordance witha preferred embodiment of the invention, as deployed for inspection of afull-sized tractor-trailer 60 while FIG. 5B shows the same cargocontainer inspection system 20 deployed for inspection of a passengervan 62. The angle of elevation 64 of the 43° scanning beam can bechanged depending upon the application. For optimum versatility, therange of limiting angles extends from at least 55° below horizontal to55° above horizontal. This corresponds to an angular adjustment of thesource axis from −33.5° to +33.5°.

Operationally, one side of a large truck 60 (up to 14′ height), asdepicted in FIG. 5A, can be completely covered in three passes; however,in many cases, satisfactory coverage can be achieved in two passes. Thesystem operators must choose between doing a third pass or tolerating asmall amount of comer cutoff 66, in which case, higher inspectionthroughput can be achieved. Since the scanning system is bi-directional,alternate passes can be in the forward and reverse directions.

Operationally, one-side of passenger cars and small trucks is scanned ina single pass of the system. Depending upon the situation, it may benecessary to scan the opposite side as well. The upper set 46 ofbackscatter detectors can be deployed over the top of smaller vehiclesas shown in FIG. 5B, substantially improving the scatter collectionefficiency and producing higher quality images. Backscatter detectormodules 46 and 48, two are typically 6′ long and 1′ wide, and eachtypically comprises four segments.

In a preferred embodiment of the invention, the cargo containerinspection system has two scan-speed modes: nominally 3 inches/sec and 6inches/sec. The faster speed results in higher throughput, the slowermode—higher image quality. In accordance with one embodiment of theinvention, image data in either mode is acquired into a 1024×4096×12 bitimage memory and displayed onto a 1024×1024 high-resolution display viaa continuously-adjustable 12-bit-to-8-bit look-up table. Additionaldisplays can be provided to allow simultaneous viewing of more than oneimage, or, alternatively, images may be superposed or combined, as knownto persons skilled in the art.

Backscatter detectors are mounted to allow efficient collection ofscattered radiation from close to the road surface, all the way to theroof of the inspected container. A motorized mechanism enables the upperset of detectors to be deployed over a small vehicle, as shown in FIG.5B, though other means of deployment are readily apparent to personsskilled in the mechanical arts and are within the scope of theinvention.

The operator's console 52 (shown in FIG. 3), provides for the consoleoperator to control the x-ray system and display images. Various displaymonitors may be provided. One preferred embodiment has an upper displayfor transmission images, and a lower display for the correspondingbackscatter image. Similarly, various display functions may be provided:

Zoom, pan and scroll—Joystick controls allow the operator to display anypart of the image at 2× and 4× magnifications.

Continuous density expand—This contrast-enhancing feature allows theoperator to display any contiguous subset of the 12-bits (4096 digitalintensity levels) of image data over the full black-to-white range ofgrey levels on the display monitors. The implementation is through a setof 10 pre-set push buttons, along with a trackball for fine-tuning.

Edge enhancement—A mathematical algorithm sharpens the image and extendsthe effective dynamic range of the display for faster and easier imageanalysis.

Reverse video—Operators may select between positive (black-on-white) ornegative (white-on-black) image display, depending on personalpreference.

Image archiving—Operators may “mark and annotate” the images from theconsole keyboard, and store them on optical disk for future recall.

Truck 24 containing cargo container inspection system 20 is fitted witha custom-built box (or truck body) 68 (shown in FIGS. 5A and 5B)specified to accommodate the imaging equipment, and to provide supportstructures, environmental control, and electrical power distribution.

Truck 24 is provided with both front- and rear-wheel drive: Standardrear-wheel drive from the truck's engine is used for normalover-the-road travel. An alternative drive mode is powered by a low-RPMhydraulic motor to obtain the very low speeds employed for the scan. Thetwo drives are connected via a switchable gearbox to preclude thepossibility of having both active at the same time. The hydraulic motorcontrols, including speed selection, drive direction, and motionstart/stop, are located in the cab of the truck under the control of thedriver. As an additional safety feature, actuation of the truck's brakewill automatically cause disengagement of the hydraulic clutch. Asimilar arrangement using a hydraulically-powered front or rear wheeldrive is known in the art for other special applications requiring veryslow vehicle motion.

Deployable beam stop 34 is employed to assure compliance with FDAradiation safety requirements. However, the output radiation of thesystem is so low that the health and safety requirements for lowradiation levels is met only a few feet away from the source even if nobeam stop is used. Beam stop 34 uses a dense shielding material such aslead that is deployed from the end of a boom 32 that extends about 14′from the side of truck 24 at the location of the x-ray beam 28.Generation of x-rays is prevented by interlock circuits unless boom 32and beam stop 34 are properly deployed.

To stow the beam stop for road travel, the beam stop is retracted intothe hollow boom 32. Boom 32 is then rotated parallel to the truck axisand lowered into a cradle in truck box 68.

The scan motion is exceedingly slow −⅓ to ⅙ of a mile per hour. Anaudible alarm is actuated whenever the scan drive mechanism is engagedfor motion in either direction. Since this motion also coincides withx-ray generation, the audible warning also provides an “X-RAYS ON”warning. The x-ray high voltage power supply 42 is interlocked so thatit cannot be energized unless both chopper wheel 26 is up to speed andtruck 24 is in motion. This additional safety precaution ensures thatthe scanning beam will not be stationary over any one region of spacefor a long time, thus ensuring low delivered dose.

Operation will be described as it applies to the inspection of one ormore passenger cars; scanning of large vehicles will be similar, exceptthat the upper detectors do not need to be swung outboard in this case.It will also be assumed that the system will first be set up, and thatvehicles to be scanned will then be brought to it. An alternativewhereby the system is deployed beside parked vehicles or containerscalls for a minor variation of procedures.

Upon arrival at the intended inspection site, the operators will firstassure that the site is suitable: i.e., that there is a sufficient spaceavailable for system operation and that operating space can reasonablybe secured for safe operations. They will then position the truck at thestarting position for the first scan, assuring that there is sufficientroom to move the truck ahead for the required scan distance, usuallyabout 65 feet. (Scans will normally alternate, forward and back; it isalso possible to scan sequentially in the forward direction only, toscan a continuous line of parked vehicles for example, provided that thenecessary space is free.) Once positioned, the on-board generator isstarted to provide power for system operation, lighting, and a coolingunit. Operator's console 52 is powered up at this time.

The operators then manually deploy the backscatter detectors and thebeam stop using a motorized mechanism provided for that purpose. Onlythe upper set of detectors 46 deploy, as shown in FIG. 5B. The beam stopis deployed by rotating the boom into a position orthogonal to thetruck, and then lowering the beam stop out of the boom to its presetlimit. This action closes an interlock circuit that is required beforex-ray generation is possible. An x-ray tube warm-up sequence, ifnecessary, is then initiated from operator console 52.

Following warmup, the physical configuration of the system setup iscompleted by rotating the x-ray beam angle to the direction (elevation)required for the intended scan operations. This is done by amanually-actuated electric motor, and with the aid of an indicator gaugeto assist in setting the desired scan elevation. Scanning operations canthen commence.

Scan operations are simple and straightforward. One or more vehicles aredirected to positions along the scan path (up to 65 feet of totalvehicle length may be imaged in a single scan) and the drivers andpassengers exit the vehicles.

Using menu-driven software, the system's computer is readied for imageacquisition. This places the computer and data acquisition electronicsinto a status wherein c-rays will be initiated and image acquisitionstarted upon receipt of a “scan” command initiated by the system'sslow-speed drive controls. The computer also transmits a “ready” statussignal to the scan drive control located next to the driver of truck 24.

The driver sets the desired scan speed and direction at the scan drivecontrol. After the “ready” status is received from the computer, thedriver starts the scan by pushing a “start” button and releasing thetruck brakes. “Start” initiates motion via the slow-speed drive. Thedriver has continuous control over the truck. He is responsible forsteering, and may stop the truck at any time by actuating the brake.Otherwise, the scan will stop automatically after a full data set hasbeen acquired by the computer (and the “ready” status is removed).

As soon as the truck is in motion, a “scan” signal is sent to thecomputer. The computer then triggers the x-ray generator to ramp up toits pre-set operating conditions, and upon confirmation that they havebeen reached (about 5 seconds later) it starts data acquisition. Dataacquisition continues until either the “scan” command is interrupted orthe image memory is full.

To break the system down for transportation, electronic systems are shutdown and the beam stop and detector mechanisms are retracted and securedfor travel. The hydraulic drive is disengaged and its power shut off.The generator is switched off.

In a further embodiment, preferred in various applications, theintensity of the transmitted x-ray beam may be measured by a single,elongated transmission detector located on the opposite side of theinspected object from the x-ray source and carefully aligned with theplane of the x-ray beam. The detector is designed to accept and respondto x-rays striking anywhere along the length of its linear entranceslit. The detector is oriented so that the flying-spot beam sweepsrepetitively from end-to-end along the slit while truck 24 moves pastthe inspected object. The detected signal is amplified, integrated,sampled and digitized into an image memory over many sequential, shorttime intervals during each sweep of the pencil beam. Each such digitizedsample forms one pixel of the final image, and the series of samplesacquired during one sweep of the beam constitutes one line of imagedata—typically 1024 samples per line. A complete image frame isconstructed by acquiring successive lines as the object is moved throughthe scan plane.

In the flying beam mode, the positional image information is acquired bycorrelating the instantaneous detector output with the position of theflying-spot beam at that instant of time. In a corresponding “fanbeam”system an entire line is illuminated at once and individual pixels alongthe line are acquired either by a large number of discrete detectorsarranged along the line, or one or more detectors with positionalsensitivity. The transmission detector may comprise scintillatorsoptically coupled to photomultiplier tubes. This method is moreefficient and less noisy than using a photodiode array. The resultingimprovement in signal/noise allows equivalent images to be made at lowerdoses and with lower beam energies.

The described embodiments of the invention are intended to be merelyexemplary and numerous variations and modifications will be apparent tothose skilled in the art. All such variations and modifications areintended to be within the scope of the present invention as defined inthe appended claims.

What is claimed is:
 1. A device, for inspecting a cargo container with penetrating radiation, the device comprising: a. a bed that is reversibly moveable along a direction having a horizontal component; b. a source of penetrating radiation, mounted on the bed for providing a beam having a central axis, the central axis being predominantly horizontal; c. a motorized drive for moving the bed in the first direction; d. at least one scatter detector mounted on the bed, each scatter detector having a signal output; so that, as the bed is moved forward and backward along the direction, the beam is caused to traverse the cargo container as the bed is moved and each scatter detector provides a signal for characterizing the cargo container and any contents of the cargo container.
 2. A device according to claim 1, wherein the a source of penetrating radiation is characterized by a source axis adjustable over a range of angles about the horizontal.
 3. A device according to claim 1, further including a remotely operated actuator for setting a desired x-ray beam angle.
 4. A device according to claim 1, further including an interlock for disabling the source of penetrating radiation unless the bed is in motion.
 5. A device according to claim 1, wherein the scatter detector is a backscatter detector.
 6. A device according to claim 1, wherein the beam is a pencil beam scanned repeatedly about an axis orthogonal to the source axis.
 7. A method for producing an x-ray scatter image of a cargo container, the method comprising: (a) providing a device having: (i) a bed reversibly moveable along a horizontal direction; (ii) a source of penetrating radiation, mounted on the bed, for providing a beam characterized by an adjustable beam axis; (iii) a motorized drive for moving the bed in the first direction; and (iv) at least one scatter detector mounted on the bed and having a signal output; (b) using the motorized drive to move the device past the cargo container so as to cause any contents of the cargo container to be scanned by the beam; and (c) processing the signal from the signal output of the scatter detector as the bed is moved forward and backward along the horizontal direction to form an image of the contents of the cargo container. 